This invention relates to improvements in the manufacture of charge coupled devices, and particularly to improvements designed to reduce the size and complexity of such devices that are constructed with two-phase overlapping gate electrodes and operate with unidirectional charge flow.
It is known to construct charge coupled devices that operate with charge flowing in one direction. One such device is described in Electronics, June 21, 1971, on pages 58 and 59. In that device the unidirectional aspect is produced by fabricating each gate electrode to have a different capacitance from its neighboring gate electrode. These two laterally adjacent gate electrodes are connected together to form one phase with an asymmetrical potential well beneath it. The next two neighboring gate electrodes of different capacitance are connected to form the other phase with an asymmetrical well beneath it, and so on.
Such a two-phase charge coupled device requires the interconnection of two adjacent gate electrodes to form a single asymmetrical potential well that specifies the direction of charge flow. This poses formidable fabrication problems in constructing a device with overlapping gate electrodes wherein two sets of the overlapping gate electrodes are arranged in two different levels. In order to connect adjacent gate electrodes, it is necessary to form interconnections between the two levels.
These problems are magnified when the memory bits or charge storage elements are arranged in serpentine fashion, because of the necessity of making cross-over connections between the rows of bits. That is, because the directionality in the flow of charge between bits in a given row is effected through the interconnection of adjacent gate electrodes, in order to direct the flow of charge along a serpentine course it is necessary to reverse the interconnections to make a transition from one row to the next, thereby causing the interconnections to cross over one another.